Foam materials are well known in the art. Foam materials typically include a solid continuous phase which comprises struts, as well as cells. The cells may comprise a continuous phase as occurs in bicontinuous phase open cell foams.
The foams useful with the present invention may relate to relatively thin, collapsed (i.e. unexpanded), polymeric foam materials that, upon contact with aqueous body fluids, expand and absorb such fluids. These absorbent polymeric foam materials comprise a hydrophilic, flexible, nonionic polymeric foam structure of interconnected open-cells that provides a specific surface area per foam volume of at least about 0.025 m.sup.2 /cc. The foam structure has incorporated therein at least about 0.1% by weight of a toxicologically acceptable, hygroscopic, hydrated salt. In its collapsed state, the foam structure has an expansion pressure of about 30,000 Pascals or less. In its expanded state, the foam structure has a density, when saturated at 88.degree. F. (31.1.degree. C.) to its free absorbent capacity with synthetic urine having a surface tension of 65.+-.5 dynes/cm, of from about 10 to about 50% of its dry basis density in its collapsed state.
While specific examples will vary, experience has shown that the foam material must be generally able to acquire aqueous fluids of nominal surface tensions against a total pressure (desorption plus gravitational) of at least about 40 cm, suitably at least about 50 cm, more suitably at least about 60 cm, and most suitably at least about 70 cm.
The overall capacity of the foam material is also quite important. While many materials such as fibrous webs may be densified so as to acquire fluids against a total pressure of about 40 to 70 cm, the capacity or void volume of such components is poor, typically less than about 2-3 g/g at 40 cm. Densification also decreases the capacity at 0 cm. Further, such webs tend to collapse under pressure (hydrostatic and mechanical) due to poor mechanical strength, further reducing their effective capacities. Even the absorbent foams described in the art for use as foam materials tend to collapse when subjected to pressures equivalent to more than about 30-40 cm of hydrostatic pressure. (Hydrostatic pressure is equivalent to mechanical pressure wherein 1 psi (7 kPa) mechanical pressure is equivalent to about 70 cm of hydrostatic pressure.) This collapse again substantially reduces (usually by a factor of between about 5 and 8) the useful capacity of these foams. While this reduced capacity can in principle be overcome by use of more absorbent material, this is generally impracticable due to cost and thinness considerations.
A third important parameter for a foam material is the ability to stay thin prior to imbibing aqueous fluids, expanding rapidly upon exposure to the fluid. This feature is described in more detail in U.S. Pat. No. 5,387,207, incorporated herein by reference. This affords a product which is relatively thin until it becomes saturated with fluid at the end of its wearing cycle. This "thin-until-wet" property is contingent upon the balancing of capillary pressures developed within the foam and foam strength, as described in U.S. Pat. No. 5,387,207.
It is believed that the ability of the polymeric foams of the present invention to remain in a collapsed, unexpanded state is due to the capillary pressures developed within the collapsed foam structure that at least equals the force exerted by the elastic recovery tendency (i.e., expansion pressure) of the compressed polymer. Surprisingly, these collapsed polymeric foam materials remain relatively thin during normal shipping, storage and use conditions, until ultimately wetted with aqueous body fluids, at which point they expand. Because of their excellent absorbency characteristics, including capillary fluid transport capability, these collapsed polymeric foam materials are extremely useful in high performance absorbent cores for absorbent articles such as diapers, adult incontinence pads or briefs, sanitary napkins, and the like. These collapsed polymeric foam materials are also sufficiently flexible and soft so as to provide a high degree of comfort to the wearer of the absorbent article.
Such relatively thin, collapsed polymeric foam materials are obtainable by polymerizing a specific type of water-in-oil emulsion having a relatively small amount of an oil phase and a relatively greater amount of a water phase, commonly known in the art as High Internal Phase Emulsions or "HIPE." The oil phase of these HIPE emulsions comprises from about 67 to about 98% by weight of a monomer component having: (a) from about 5 to about 40% by weight of a substantially water-insoluble, monofunctional glassy monomer; (b) from about 30 to about 80% by weight of a substantially water-insoluble, monofunctional rubbery comonomer; (c) from about 10 to about 40% by weight of a substantially water-insoluble polyfunctional crosslinking agent component. The oil phase further comprises from about 2 to about 33% by weight of an emulsifier component that is soluble in the oil phase and will provide a stable emulsion for polymerization. The water or "internal" phase of these HIPE emulsions comprises an aqueous solution containing from about 0.2 to about 20% by weight of a water-soluble electrolyte. The weight ratio of the water phase to the oil phase in these HIPE emulsion may range from about 12:1 to about 100:1. The polymerized foam is subsequently dewatered (with or without prior washing/treatment steps) to provide the collapsed foam material.
The foam material may be used as a storage element in an absorbent article. An important characteristic of the storage element is the ability to wick fluid within itself. Wherein the overlap between an acquisition or distribution component and the storage element is only partial, the storage component must itself be able to wick fluid throughout itself to be efficient.
It is also desirable that the storage element be sufficiently tough to survive during use and manufacture, sufficiently flexible to be comfortable, and amenable to manufacture using commercially viable procedures for large scale production.
Various techniques have been attempted in the art to remove fluids indigenous to manufacture of the foam material. For example, evaporative drying under ambient conditions (while not requiring a significant capital outlay) does not yield a drying rate which economically produces foam materials. Infra-red drying of foam materials requires expensive equipment and may produce moisture gradients in large quantities of the foam materials--thereby destroying any economies of scale. Thus, the foam materials must be economically dried.
Furthermore, such foams must be dried to the proper moisture level. If regions in the foam are overdried, random and uncontrolled swelling of such regions may occur. Such random swelling makes it difficult to reliably incorporate the foam absorbent materials into consumer products. Further, such random swelling makes it difficult to predict the ultimate performance of such foam materials at the point of use by the consumer. Thus, the foam materials must be uniformly dried to the proper moisture level.
Accordingly, there exists a need in the art for processes to economically dry foam materials, particularly foam absorbent materials, high internal phase emulsion foams, and other foams having relatively small-sized capillary networks. Further, there exists a need in the art to uniformly dry relatively large quantities of such materials. Finally, there exists a need in the art to uniformly dry such materials to a desired moisture level.